Running FSL on new Matlab version on macOS

### Description

@mervinTEE didn't get ExploreASL to run FSL.


### Tasks

- [x] Henk did a minor bugfix for loading 'ascii' data
 
### How to test
```
x=ExploreASL;
[~, x] = xASL_fsl_SetFSLdir(x, 1);
xASL_fsl_RunFSL('fsleyes', x);
```

### Release notes


#1458 Minor bugfix running FSL on newer Matlab versions